This invention relates to improvements in an elevator traction machine, especially to improvements in the grounding system.
FIG. 1 shows a conventional elevator traction machine driven by an electric motor controlled by a thyristor Leonard system. In FIG. 1, reference characters R, S and T designate a three-phase AC supply. An armature 2a of a DC motor M (the field not shown) has the applied voltage controlled by a semiconductor switching circuit 1, such as a thyristor converter. The motor M has an electrical conductive shaft 3. The shaft 3 is supported by electrically conductive bearings 7, 8 that are supported by electrical conductive bearing stands 71, 81 disposed on an electrically conductive machine beam 4. The machine beam 4 is set through vibration-proof, non-conductive rubber members 5 in a machine room.
A grounding line 6 is used to ground the machine beam 4. This is in accordance with Japanese Industrial Standards for Electrical Equipment Installation. Accordingly, the shaft 3 is grounded through the bearings 7, 8, the bearing stands 71, 81, the machine beam 4, and the grounding line 6. A conductive traction sheave 9 is fixedly mounted on the shaft 3. A deflection sheave 10 is mounted on and fixedly secured to a shaft 11. Bearings 12 and 13 support the shaft 11 on the machine beam 4. A main rope 14 is wound on both the traction sheave 9 and the deflection sheave 10. A cage 15 and a counterweight 16 are connected to the main rope 14 at respective ends thereof. The shaft 3 is also grounded through the traction sheave 9, the main rope 14, the deflection sheave 10, the shaft 11, the bearings 12, 13, the machine beam 4, and the grounding line 6.
A stray capacitance 17 may be formed between the armature 2a and the shaft 3 thereof and stray capacitances 18, 19 and 20 may be formed between the supply lines L.sub.R, L.sub.S, L.sub.T and ground. This is well known in the art.
It is well known in the art that, when an electric motor is controlled by a high-speed switching element such as a thyristor, then shaft current flows in the electric motor. More specifically, when the thyristor convertor 1 is operated, a pulsive current flows through a circuit ##STR1## and/or a different circuit ##STR2## In general, the bearings 7, 8, 12 and 13 are ball-and-roller bearings. Therefore, when the shaft current flows in these bearings, the surfaces of the balls or the rollers are electrolytically corroded, and the balls or the rollers are quickly damaged.
In order to prevent or lessen the flow of the shaft current through the bearings 7, 8, 12 and 13, a current collector 22 is provided, as shown in FIG. 2. The current collector contacts with the shaft 3 of the electric motor 2 and is grounded through a grounding line 23.
However, this method is disadvantageous in that the current flowing in the bearings 7, 8, 12 and 13 is not significantly decreased depending on the contact resistance of the current collector 22. Also, maintaining the contact resistance at desired low value is troublesome.